Aqueous-phase Reaction Mechanisms Of Methylglyoxal/glyoxal And Typical Nitrogen-containing Compounds In The Atmosphere | | Posted on:2022-11-30 | Degree:Master | Type:Thesis | | Country:China | Candidate:L Gao | Full Text:PDF | | GTID:2491306779497964 | Subject:Environment Science and Resources Utilization | | Abstract/Summary: |  PDF Full Text Request | | Brown carbon（Br C）is a kind of important light-absorbing organic aerosol（OA）,which has a profound impact on atmospheric radiative forcing,global climate and human health.Compared with the primary sources,the secondary sources of Br C involve more complex heterogeneous reactions and have more uncertainties.In the atmosphere,smallα-dicarbonyl compounds have wide sources and high concentrations.Their aqueous-phase reactions with nitrogen-containing compounds are considered to be an effective and important source of secondary Br C.Most of previous studies focused on the sources,composition and optical properties of secondary Br C,but the aqueous-phase reaction mechanisms of different smallα-dicarbonyl compounds with nitrogen-containing compounds are still unclear,which affects the assessment of the chemical properties,composition,optical properties and environmental health risks of secondary Br C.Therefore,in this study,the aqueous-phase reaction mechanisms and the contribution to secondary Br C formation of smallα-dicarbonyl compounds with nitrogen-containing compounds were systematically investigated by using quantum chemistry and kinetic calculation.Glyoxal（GL）and methylglyoxal（MG）were selected as the representatives of typical smallα-dicarbonyl compounds;ammonia（AM）as inorganic nitrogen-containing compound,methylamine（MA）/dimethylamine（DA）/Glycine（GLY）as organic nitrogen-containing compounds,were chosen as the representatives to investigate the possible aqueous-phase reactions.The aqueous-phase reaction mechanisms and kinetics of smallα-dicarbonyl compounds with nitrogen-containing compounds were clarified,and the mechanisms of the effect of methyl group on the light absorptivity along with the health risks of Br C were revealed.The research results are detailed as follows:（1）In acidic aqueous-phase environment,the proton-mediated indirect association reactions between MG and AM/MA/DA initiated the reactions between MG and organic and inorganic nitrogen-containing compounds.For the MG+AM reaction system,oligomerization pathways that mediated by amino alcohols,imines and diimines possess similarly low energy barriers and small changes on reaction energies,which indicates that imines and diimines are the key intermediates in the formation of nitrogen-containing heterocycles.However,for the MG+MA reaction system,the amino alcohols/imines-mediated oligomerization pathways are more efficient to generate nitrogen-containing heterocycles.For the MG+DA reaction system,only nitrogen-containing chain oligomers were yielded because of the two methyl groups in DA,indicating that the amount of methyl groups in nitrogen-containing compounds determine the formed Br C types.The analysis of predicted UV-Vis absorption spectra shows that the imidazole nitrogen-containing heterocycles exhibit red-shift in the maximum absorption peak and the stronger light absorptivity than non-imidazole nitrogen-containing heterocycles and nitrogen-containing chain products.The health risk assessment results indicate that the imidazole nitrogen-containing heterocycles possess more adverse effects on the cardiovascular system,gastrointestinal system and lung;while the nitrogen-containing chain products have more adverse effects on the cardiovascular system.In summary,the increase of methyl groups in the nitrogen-containing compounds diminishes the absorbance intensity of the formed Br C.（2）For the GL/MG+GLY reaction systems,under acidic aqueous-phase conditions,GL and MG are easily converted into the diol forms,which readily yield a large amount of carbenium ions.The thermodynamic results prove that the subsequent nucleophilic addition reactions of carbenium ions with GLY are the dominantly initial reactions.And the imines and diimines are formed subsequently,which continue to undergo oligomerization reactions to yield nitrogen-containing heterocycles eventually.For the GL+GLY reaction system,the energy barrier of the cyclization process in the diimine-mediated oligomerization pathway is～12 kcal mol-1 higher than that in the imine-mediated oligomerization pathway;for the MG+GLY reaction system,the energy barrier of the cyclization process in the diimine-mediated oligomerization pathway is 5～12 kcal mol-1 higher than that in the imines-mediated oligomerization pathways.It can be concluded that imines-mediated oligomerization pathways dominate the nitrogen-containing heterocycles formation in GL/MG+GLY reaction systems.In addition,it can be found that more kinds of imines and carbenium ions were yielded in MG+GLY reaction system owing to the methyl group in MG,which results in the formation of more types of nitrogen-containing heterocycles.The predicted UV-Vis absorption spectra of the main products shows that the products in GL+GLY reaction system exhibit stronger light absorptivity than that in MG+GLY reaction system.The results can be attributed to the conjugation system in the products of GL+GLY reaction system,whlie no conjugation system was found in the products of MG+GLY reaction system because of the methyl substituent in MG.Therefore,the presence of methyl groups in the aqueous-phase reaction of smallα-dicarbonyl compounds diminishes the intensity of the absorbance of the products. | | Keywords/Search Tags: | small α-dicarbonyl compounds, aqueous-phase reactions, reaction mechanisms, brown carbon, light absorptivity | | PDF Full Text Request | Related items |
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